This invention relates to ohmic contacts and semiconductor bodies and more particularly to an improved multilayer low resistance solderable contact that can be used on both N-type silicon and P-type silicon.
In the past, nickel layers have been used as single layer solderable ohmic contacts directly on N-type silicon. In such contacts, the nickel layer is applied by electroless deposition from an aqueous solution containing nickel sulfate and sodium hypophosphite. The plated silicon body is heated after the nickel is deposited. After heating at a moderate temperature, the nickel layer has a low contact resistance on N-type silicon. This is due to a significant phosphorus concentration in the nickel layer. However, the phosphorus concentration that reduces contact resistance on N-type silicon, increases it on P-type silicon. Hence, for lowest resistance solderable ohmic contacts on P-type silicon, other aproaches have been used.
Excellent layer resistance contacts are regularly made to P-type silicon with a specially microalloyed aluminum layer. However, aluminum is not readily solderable. It is generally known to coat aluminum with one or more layers of another metal, to provide an outer layer that is solderable. Various metals and deposition techniques can be used in making semiconductor devices, vacuum deposition is frequently used. Coatings of pure nickel can be conveniently applied to aluminum by a vacuum deposition. Pure nickel provides a highly solderable surface and does not introduce undesirable impurities to the semiconductor surface. However, the adhesion of pure nickel to aluminum is unsatisfactory. It is not as strong as the aluminum-silicon bond, or the nickel-solder bond.
We have found that it is as difficult to get pure nickel to satisfactorily adhere to aluminum as it is to get solder to do so. For example, when a silicon element having an aluminum-pure nickel multilayer contact is soldered to a supporting substrate and subjected to bending stresses, the nickel separates from the aluminum to produce electrode failure.
In our U.S. Pat. No. 3,886,585 entitled "Solderable Multilayer Contact for Silicon Semiconductor" we disclosed that a vacuum deposited manganese-nickel alloy adhered better than pure nickel to an aluminum surface. Moreover, the manganese-nickel alloy did not introduce undesirable impurities to the semiconductor surface, increase the number of processing steps, or reduce solderability.
Such an alloy is readily deposited by sputtering, and by more sophisticated electrical resistance evaporation techniques. However, it is not so readily deposited, particularly under commercial production conditions, with a multiple source electron beam evaporation apparatus or the more rudimentary filament type electrical resistance heated evaporation apparatus. Vapor pressure difference between the nickel and manganese can produce control problems with the latter apparatus that are objectionable for commercial production operations. A given commercial production facility may have a substantial capital investment in this latter vacuum deposition apparatus. If so, it may not be economically prudent to convert to sputtering apparatus or to a more sophisticated electrical resistance heated evaporation apparatus. In such instance there may be a reluctance to use our prior invention in that given production facility.
On the other hand, we have found that the manganese and nickel need not be simultaneously evaporated as an alloy. Extremely adherent solderable contacts can be obtained by successively evaporating the manganese and nickel in a single evacuation step. High yields can be consistently obtained under commercial production conditions with the multiple source electron beam heated evaporation apparatus and with the more rudimentary filament type resistance heated evaporation apparatus referred to earlier. Also, sputtering apparatus or more sophisticated electrical resistance heated evaporation apparatus can be used, but are preferably used with our invention disclosed in the aforementioned U.S. Pat. No. 3,886,585.